76 Minimum Window Substring

🔀 Pick One

Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

The testcases will be generated such that the answer is unique.

A substring is a contiguous sequence of characters within the string.

Example 1:

Input: s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"
Explanation: The minimum window substring "BANC" includes
'A', 'B', and 'C' from string t.

Example 2:

Input: s = "a", t = "a"
Output: "a"
Explanation: The entire string s is the minimum window.

Example 3:

Input: s = "a", t = "aa"
Output: ""
Explanation: Both 'a's from t must be included in the window.
Since the largest window of s only has one 'a', return empty string.

Constraints:

  • m == s.length
  • n == t.length
  • 1 <= m, n <= 105
  • s and t consist of uppercase and lowercase English letters.
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class Solution:
    def minWindow(self, s: str, t: str) -> str:
        need = collections.Counter(t)
        missing = len(t)
        start, end = 0, 0
        i = 0
        for j, c in enumerate(s, 1):   # index j from 1
            if need[c] > 0:
                # included a char in t
                missing -= 1
            need[c] -= 1
            if missing == 0:
                # match all chars, advance left pointer
                while i < j and need[s[i]] < 0:
                    need[s[i]] += 1
                    i += 1
                # make sure the first appearing char satisfies need[char]>0
                need[s[i]] += 1
                # we missed this first char, so add missing by 1
                missing += 1
                if end == 0 or j-i < end-start:
                    start, end = i, j
                i += 1
        return s[start:end]